1. Field of the Invention
The present invention relates to a sizing composition for glass threads, this composition reacting to ultraviolet radiation or to an electron beam. The present invention relates also to a process for the production of reinforcing glass threads using this composition, and also to the glass threads obtained and the composites produced starting from the threads.
2. Discussion of the Background
In the course of the text, "polymerisable", "polymerise", and "polymerisation" are to be understood as meaning, respectively, "polymerisable and/or crosslinkable", "polymerize and/or crosslink", and "polymerization and/or crosslinking". Likewise, "under U.V. or E.B." and "U.V. irradiation or E.B. exposure" are to be understood as meaning, respectively, "under the action of ultraviolet radiation or of an electron beam" and "irradiation by ultraviolet radiation or exposure to an electron beam".
The manufacture of reinforcing glass threads is effected in known manner starting from thin streams of molten glass flowing from the orifices of die plates. These thin streams are drawn in the form of continuous filaments and then the filaments are gathered into basic threads which are subsequently collected.
Before they are gathered in the form of threads, the filaments are coated with a size by being passed over a sizing member. The depositing of size is necessary in order to obtain the threads and enables the threads to be associated with other organic and/or inorganic materials in order to produce composites.
The size acts primarily as a lubricant and protects the threads from the abrasion resulting from the high-speed friction of the threads on various members during the above-mentioned process.
The size can also ensure, especially after polymerisation, the integrity of the above-mentioned threads, that is to say, the interconnection of the filaments within the threads. This integrity is especially desired in textile applications where the threads are subjected to great mechanical stresses. If the filaments are poorly integral with one another they break more easily and interfere with the operation of the textile machines. In addition, threads that do not exhibit integrity are considered to be difficult to handle.
The size also facilitates the wetting and/or impregnation of the threads with the materials to be reinforced and contributes to the creation of bonds between the said threads and materials. In particular, the mechanical properties of the composites obtained from the said materials and threads depend on the quality of the adhesion of the material to the threads and on the suitability of the threads for being wetted and/or impregnated by the material.
The sizing compositions used have to be sufficiently stable, especially under the die plate, and compatible with the drawing speeds of the filaments that have to pass through them (up to several tens of meters per second). In particular, they must be able to withstand the shearing induced by the passage of the filaments and must wet the surface of the filaments at the said speeds. It is also desirable for these compositions to exhibit a maximum degree of conversion after polymerisation (this degree corresponding to the ratio between the number of reactive groups that have reacted in the size after polymerisation and the number, in the size, of reactive groups that are capable of reacting, before polymerisation) in order to ensure, in particular, that glass threads having a constant quality are obtained (a size exhibiting a degree of conversion much lower than the expected theoretical degree being capable of changing over time).
The majority of sizes currently used are aqueous sizes which are easy to handle but which have to be deposited on the filaments in large quantities in order to be effective. Water generally constitutes 90% by weight of these sizes (in particular for reasons of viscosity), which makes it necessary to dry the threads before they are used as reinforcements, since the water may impair the good adhesion between the threads and the materials to be reinforced. These drying operations are long and expensive, have to be adapted to the manufacturing conditions of the threads and their efficacy is not always optimum. In addition, when they are carried out on rolls of threads, they entail the risk of irregular and/or selective migration of the components of the size within the rolls (aqueous sizes already having a tendency to spread over the threads in an irregular manner, owing to their nature) and, in certain cases, the phenomena of the threads' becoming coloured or the rolls' being deformed. The deformation of the rolls is also observed, in the absence of drying, on straight-edged rolls (rovings) of "fine" threads (that is to say, threads having a "titre" or "linear mass" of from 300 to 600 tex (g/km) or less) coated with aqueous sizes.
A few patents describe non-aqueous sizes, but these sizes generally use organic solvents which are tricky to handle and may impair the health of persons in the vicinity because they are toxic, and/or pose viscosity problems which are
solved by heating the sizes or by adding suitable agents.
Patent EP-B1 243 275 describes a size comprising at least one mono- or poly-unsaturated oligomer and a photo-initiator (of the free radical type), which size reacts to ultraviolet radiation and is used in a particular process. According to this process, the size is deposited on the filaments before they are gathered into threads, and then the threads are subjected to ultraviolet radiation, before being wound, in such a manner as to polymerise the size and prevent adhesion between the turns of each roll, which would make it difficult to unwind the threads. Such a process results in threads that exhibit integrity and that can be handled directly. However, the polymerised size prevents the filaments from sliding relative to one another, this lack of mobility causing the threads to burst when cut owing to the mechanical degradation of the size and possibly creating problems in textile applications where the threads used have to exhibit both integrity and pliability (the pliability of the threads being linked with the pliability of the size and the amount of size deposited). In addition, the loss on ignition of the threads obtained is rather high and the degree of conversion after polymerisation hardly exceeds 75% .
Another size (based on acrylates, N-vinylpyrrolidone and a free radical photo-initiator) reacting to ultraviolet radiation is mentioned in patent EP 570 283 which describes a process in which the irradiation of the threads is carried out on rolls of thread during the winding operation. The glass threads obtained exhibit good mechanical characteristics, a satisfactory degree of conversion owing to the longer irradiation time and a low loss on ignition, but their integrity is not entirely satisfactory. However, it seems hardly economical to increase the amount of size deposited on the threads (therefore loss on ignition) in order to improve integrity and that could result in sized threads having too high a degree of rigidity.